Webapp: Fix error messages during upstream creation

[saarasio]

Note that when upstream_ip is not provided, the error message is - "No JSON object could be decoded". This is not very helpful.

[ec2-user@ip-172-31-18-10 enroute-cp]$ curl -s -X POST localhost:1323/upstream -H "Authorization: Bearer treehugger" -H "Content-Type: application/json" -d '{"Upstream_name":"db_serve_process", "upstream_hc_host":"127.0.0.1", "upstream_hc_path":"/v1/version", "upstream_port":"8081"}' | python -m json.tool
No JSON object could be decoded
[ec2-user@ip-172-31-18-10 enroute-cp]$ curl -s -X POST localhost:1323/upstream -H "Authorization: Bearer treehugger" -H "Content-Type: application/json" -d '{"Upstream_name":"db_serve_process", "upstream_hc_host":"127.0.0.1", "upstream_hc_path":"/v1/version", "upstream_port":"8081"}' | python -m json.tool
No JSON object could be decoded
[ec2-user@ip-172-31-18-10 enroute-cp]$ curl -s -X POST localhost:1323/upstream -H "Authorization: Bearer treehugger" -H "Content-Type: application/json" -d '{"Upstream_name":"db_serve_process", "upstream_ip":"127.0.0.1", "upstream_hc_path":"/v1/version", "upstream_port":"8081"}' | python -m json.tool
{
    "Error": "Please provide a valid weight value."
}
[ec2-user@ip-172-31-18-10 enroute-cp]$ curl -s -X POST localhost:1323/upstream -H "Authorization: Bearer treehugger" -H "Content-Type: application/json" -d '{"Upstream_name":"db_serve_process", "upstream_ip":"127.0.0.1", "upstream_hc_path":"/v1/version", "upstream_port":"8081", "upstream_weight":"98"}' | python -m json.tool
{
    "data": {
        "insert_saaras_db_upstream": {
            "affected_rows": 1
        }
    }
}